Spectrophotometers for analyzing the composition of materials have been widely known wherein the sample to be tested is illuminated with a monochromatic radiation of variable wavelength at a standard 0.degree. or 45.degree. angle of incidence, and the intensity of the radiation scattered from the sample at an angle of 45.degree. or 0.degree., respectively, is measured. The standard lay outs of 0.degree./45.degree. and 45.degree./0.degree. are intended to eliminate the measuring of the "mirrored" radiation reflected normally from the surface of the sample and ensure the measuring of the "scattered" radiation, only. A common feature of the known devices is that only a small fraction of the radiation scattered back from the sample at the required angle reaches the sensor, as the solid state sensors generally used are usually made with small dimensions and flat sensitive surface because of technological reasons.
This holds true even to a greater extent in case of some samples to be tested, such as grain milling products, fodder blends, meat products, mashed fruits and vegetables, etc., which are more or less inhomogeneous and therefore large exposed surface of the sample to be tested and correspondingly large cross-sectional area of the illuminating radiation should be provided for. From these large sample surfaces even a smaller fraction of the radiation scattered symmetrically in all directions at a predetermined angle with respect to the normal of the sample surface will reach the radiation sensor in the devices known heretofore. In order to utilize at the sensitive surface of the sensor as large a portion of the radiation scattered from the sample at a predetermined angle as possible, several radiation sensors are applied. Thereby the energy reaching the sensitive surface of the sensors can be increased but the density of the radiating energy will remain small, this latter fact being unfavourable considering measuring accuracy. The lower the density of the radiating energy, the smaller the signal provided by the sensor will be, and thus the signal to noise ratio will deteriorate and the time-constant of the sensor will increase.